Formation of the blood vasculature depends on the precise control of molecular networks that are tightly regulated by proangiogenic growth factors and by molecules involved in endothelial cell (EC) metabolism. However, the transcriptional control of these processes remains incompletely understood. The long-term goal of our lab is to elucidate the fundamental mechanisms that regulate the formation of blood vessels and to understand how the disruption of these mechanisms leads to vascular defects in pathological settings. FOXC1 and FOXC2 are closely related members of the FOX (Forkhead box) transcription factor family and have critical roles in vascular development and disease. Mutations or changes in the copy number of human FOXC1 are associated with autosomal-dominant Axenfeld-Rieger syndrome (ARS), which is characterized by anterior eye segment defects and cerebral small vessel disease, while inactivating mutations of FOXC2 are responsible for autosomal-dominant lymphedema-distichiasis syndrome, which includes symptoms such as late-onset lymphedema and extra eyelashes (distichiasis). We have completed preliminary experiments suggesting (1) that the mutations are associated with declines in filopodia formation and proliferation at the angiogenic front, defects in vascular density and branching in the capillary plexus, and impaired vascular patterning; and (2) that these angiogenic defects are accompanied by significant declines in the activity of mammalian target of rapamycin (mTOR) and in the expression of CD98, which imports essential amino acids such as leucine while exporting the nonessential amino acid glutamine. Thus, our central hypothesis is that the Foxc transcription factors participate in physiological and pathological angiogenesis by regulating pathways involved in amino acid transport, EC metabolism, and mTOR signaling. Guided by strong preliminary data, this hypothesis will be tested by pursuing two specific aims: 1) identify the molecular and genetic networks that link the Foxc transcription factors with angiogenesis and EC metabolism; 2) Define the mechanisms by which Foxc1 and Foxc2 cooperatively participate in physiological and pathological angiogenesis. In summary, the results generated from the experiments described in this proposal will provide crucial information about the formation of blood vessels; thus, because vascular deficiencies are among the leading causes of cardiovascular disease and disorders, our findings are likely to identify new targets and therapeutic strategies for improving vascular formation and function in affected patients.